This invention relates to nozzles for a turbine and particularly to a technique for attaching a cover to nozzle having angel wing seals.
Turbine nozzles in a gas turbine are typically attached to an inner cylindrical surface of a turbine casing. The nozzles form a radial array extending inward towards the turbine shaft. Each nozzle array is a stationary row arranged between rows of rotating turbine buckets. Hot combustion gases flow through the rows of nozzles and buckets to drive the rotation of the buckets. The rows of nozzles turn the hot combustion gases to align with the leading edges of the row of buckets that are immediately downstream of the nozzle row.
The hot gasses flow through an annular flow path across the nozzles and buckets. Gases are prevented by seals from flowing out of the flow path and into the wheel cavities. The seals include angel wings that are arms extending outward from the buckets towards the nozzles. The angel wings on a row of buckets form rings that overlap the edges of the cover plates on each of the nozzles in adjacent front and aft nozzle rows. The nozzle cover plates have an angel wing seal land that is overlapped by the angel wings on the buckets. The gap between the angel wings and the seal land is relatively small to minimize hot gas path flow leakage from the flow path to the wheel cavities.
During fabrication of each nozzle, cover plates are welded to the inner and outer bands of the nozzle castings to form a closed loop cooled nozzle. The nozzle casting consists of the nozzle blade and the inner and outer bands. The cover plates are typically attached to the nozzle casting by welding. Vertical welds are used to join the cover plates to the nozzle casting. The vertical weld of the cover plates to the nozzle casting increases the low-cycle fatigue (LCF) life of the nozzle casting to cover plate weld joint by allowing the joint to be placed in a region of low thermal gradient.
One source of difficulty in welding of the cover plates to a turbine nozzle casting has been the angel wing seal land that is typically a radial ridge across one end of the cover. The surface of the angel wing seal land is smooth and radial because it is to be in close proximity to the rotating angel wings of the buckets.
A continuous angel wing seal land is helpful to minimize ingestion of hot flow path air into the wheel space cavity. To have a continuous seal across the nozzle sidewall, the angel wing land would cross the cover plate to nozzle casting weld joint and interfere with the cover weld joint.
To avoid the angel wing seal lands and to provide for the vertical joint weld, compromises were made to the geometry of the angel wing seal land and of the joint between the machined cover and the cast nozzle. Further, to allow proper access to the joint of the welding head the length of angel wing seal land has been shortened and then land extenders are later added to the cover. Accordingly, there is a long felt need for welded joint geometries and techniques for welding a cover with angel wing seal lands to a nozzle casting.
This invention was developed to allow for a continuous angel wing seal land on the nozzle while permitting clearance for a vertical weld of the nozzle casting to the cover plate. In one embodiment, the invention is a turbine nozzle is disclosed having a nozzle blade with an inner band, wherein the inner band has a region to receive a nozzle cover plate and at least one raised angel wing seal land section proximate a side of the recess; the nozzle cover plate is seated on the region of the inner band and the plate has a raised angel wing seal land section aligned with the at least one raised angel wing seal land section on the inner band, wherein the perimeter of the nozzle cover is welded to the inner band; a slot in the angel wing seal land is adjacent the perimeter of the cover plate, and the slot is filled with an insert after the cover plate is welded to the nozzle inner band.
In a second embodiment the invention is a nozzle for a turbine comprising: a nozzle blade having an inner band, wherein said inner band has a recess to receive a nozzle cover plate and a pair of raised angel wing seal land sections proximate opposite sides of the recess; the nozzle cover plate being seated in the recess of the inner band and said plate further comprises a raised angel wing seal land section aligned with the pair of angel wing seal land sections on the inner band, wherein an perimeter of the nozzle cover is welded to a side wall of the recess in the inner band; a pair of slots in the angel wing seal land section on the cover plate, wherein the slot is adjacent the perimeter of the cover plate, and an insert in each of the slots, wherein said insert is substantially flush with a seal surface of the angel wing seal land section on the cover plate.